The present invention relates to an electron-emitting source which is arranged in vacuum and emits electrons upon application of the voltage, and a method of manufacturing the same.
Electron-emitting sources are used in an electron gun as a constituent component of the picture tube of, e.g., a television. Such an electron gun (cathode) has a basic arrangement like the one shown in FIG. 14 in which an electron-emitting layer 1402 is formed at the closed distal end of a cathode cylinder 1401. The cathode cylinder 1401 incorporates a heater 1403.
The cathode cylinder 1401 is made of high-purity Ni doped with a reducing agent such as magnesium or silicon, and has a thickness of about 0.1 mm. The electron-emitting layer 1402 is made of a so-called ternary oxide of barium oxide, calcium oxide, and strontium oxide. When the electron-emitting layer 1402 is heated to about 800.degree. C. by the heater 1403, barium is mainly reduced and liberated. The free barium moves toward the surface of the electron-emitting layer 1402 to facilitate electron emission. Emitted electron beams are collected by a first grid 1404 and focused on a phosphor screen 1409 via a second grid 1405, a third grid 1406, a fourth grid 1407 serving as a focusing electrode, and a fifth grid 1408 (FIG. 14B).
In this manner, the electron-emitting source is used in vacuum. Such electron-emitting sources are used in not only the electron gun of the picture tube but also a vacuum fluorescent display apparatus. This display apparatus is an electron tube using emitted light obtained by bombarding electrons emitted by the electron-emitting source against the phosphor within a vacuum vessel having at least one transparent end. In many cases, the vacuum fluorescent display apparatus employs a triode structure having a grid for controlling the movement of electrons. In the vacuum fluorescent display apparatus, a cathode called a filament has conventionally been used for the electron-emitting source, and thermoelectrons emitted from the cathode are bombarded against the phosphor to emit light.
Such fluorescent display apparatuses include an image tube constituting the pixel of a large-screen display apparatus. The arrangement of an electron-emitting source used in the image tube will be described with reference to FIG. 15.
The respective components of the image tube are stored in a cylindrical glass valve 1501 constituting a vacuum vessel. The glass valve 1501 incorporates a cathode structure 1510 serving as an electron-emitting source. The cathode structure 1510 has the following arrangement. A back electrode 1512 is formed at the center of a ceramic substrate 1511. A filament cathode 1513 is fixed above the back electrode 1512 at a predetermined interval. An elliptical grid housing 1514 having a mesh portion 1514a is mounted on the ceramic substrate 1511 to cover the back electrode 1512 and the filament cathode 1513. The mesh portion 1514a spherically projects toward a phosphor screen (not shown) arranged inside the glass valve 1501.
The image tube having this arrangement emits electrons from the cathode structure 1510 in the following manner. A predetermined voltage is applied to the filament cathode 1513 to emit thermoelectrons. A negative voltage with respect to the filament cathode 1513 is applied to the back electrode 1512. A positive voltage with respect to the filament cathode 1513 is applied to the grid housing 1514. Then, an electron beam is emitted from the mesh portion 1514a of the grid housing 1514. The electron beam lands on a phosphor screen (not shown) to cause the phosphor screen to emit light.
As described above, thermoelectron emission using an electron-emitting substance is basically adopted in a conventional electron-emitting source used in an apparatus such as a picture tube or a vacuum fluorescent display apparatus using emission by the phosphor upon bombardment of electrons. The electron-emitting substance is made of a so-called ternary oxide of barium oxide, calcium oxide, and strontium oxide. Barium in this ternary oxide is consumed upon reaction with gas during use, and thus the tube is always replenished with barium from the electron-emitting layer. However, barium replenishment is insufficient even if a large current is flowed to emit many electrons. In addition, the electron-emitting substance is heated by electron emission but deteriorated by heat.
Oxides constituting the electron-emitting substance are very unstable in air. For this reason, in manufacturing a conventional electron-emitting source, an electron-emitting layer is first formed from so-called carbonates such as barium carbonate, calcium carbonate, and strontium carbonate, incorporated in a vacuum vessel together with other components, and oxidized while the vacuum vessel is evacuated and aged. Accordingly, manufacturing the conventional electron-emitting source requires many steps.
The electron flow emitted by the conventional electron-emitting source greatly depends on the temperature of the electron-emitting source. If the temperature of the electron-emitting source varies depending on the place, the electron flow also varies.
The conventional electron-emitting source is made of the electron-emitting substance, as described above. However, this substance is weak with respect to the gas produced in the vacuum vessel of the vacuum fluorescent display apparatus and may deteriorate within a short time.
In short, the conventional electron-emitting source suffers the problems of a cumbersome manufacturing process, variations in emitted current flow, low environmental resistance, and a weak structure.